1. Field of the Invention
The present invention relates to an ejector apparatus.
2. Description of the Related Art
Conventionally, in a injection molding machine, a resin heated and melted in a heating cylinder is charged into a cavity in a die apparatus at high pressure. The molten resin is then cooled and solidified in the cavity to obtain a molded product.
The die apparatus is composed of a stationary die and a movable die. A toggle mechanism advances and retracts the movable die so as to contact the movable die with and separate the same from the stationary die, thereby effecting die closing, die clamping, and die opening. During die opening, a die clamping apparatus retracts, leaving a molded product in the movable die. Subsequently, an ejector apparatus advances ejector pins so as to push the molded product out of the movable die.
To achieve the ejecting operation described above, the ejector pins are disposed such that their front ends face the cavity and that their rear ends are fixed to an ejector plate. The rear end of the ejector plate is connected to an ejector pin feed apparatus via ejector rods. When the ejector pin feed apparatus is operated so as to advance the ejector plate via the ejector rods, the ejector pins fixed to the ejector plate advance.
In a motor driven injection molding machine, the ejector pin feed apparatus is usually driven by an electric motor.
FIG. 1 is a front view of a conventional single-axis ejector apparatus, and FIG. 2 is a side view of the conventional single-axis ejector apparatus.
In FIGS. 1 and 2, reference numeral 11 denotes a movable platen. An unillustrated movable die is attached to the front end (to the right end in FIG. 1) of the movable platen 11. The movable platen 11 is advanced and retracted by a toggle mechanism 14 along tie bars 12 extending between a stationary platen and a toggle support, both not shown.
An ejector pin feed apparatus 16 is disposed at the rear end (at the left end in FIG. 1) of the movable platen 11. The ejector pin feed apparatus 16 includes guide posts 18. The front ends of the guide posts 18 are fixed to the movable platen 11, and a support member 19 is fixed to the rear ends of the guide posts 18. A ball nut 22 is rotatably supported by the support member 19 via a bearing 20.
The inner circumference of the ball nut 22 is screw-engaged with a ball screw shaft 24. A cross head 25 is fixed to the front end of the ball screw shaft 24. Further, ejector rods 27 are fixed to the front end of the cross head 25. Accordingly, by rotating the ball nut 22, the ball screw shaft 24 can be advanced and retracted in the direction of arrow A so as to advance and retract the ejector rods 27 in the same direction.
The ejector pin feed apparatus 16 is operated by a servomotor 28, which is drivingly connected to the ball nut 22 via a belt transmission mechanism 31. The belt transmission mechanism 31 comprises a pulley 32 attached to the output shaft of the servomotor 28, a pulley 33 attached to the rear end of the ball nut 22, and a timing belt 34 wound around the pulleys 32 and 33 with tension.
Accordingly, when the servomotor 28 is driven, the rotational motion of the servomotor 28 is transmitted to the ball nut 22 via the belt transmission mechanism 31. The ball nut 22 and the ball screw shaft 24 convert the rotational motion into a linear motion, so that unillustrated ejector pins are advanced and retracted.
Next, a two-axis ejector apparatus will be described.
FIG. 3 is a front view of a conventional two-axis ejector apparatus, and FIG. 4 is a side view of the conventional two-axis ejector apparatus.
In FIGS. 3 and 4, reference numeral 11 denotes a movable platen. An unillustrated movable die is attached to the front end (to the right end in FIG. 3) of the movable platen 11. The movable platen 11 is advanced and retracted by a toggle mechanism 14 along tie bars 12 extending between a stationary platen and a toggle support, both not shown.
An ejector pin feed apparatus 16 is disposed at the rear end (at the left end in FIG. 3) of the movable platen 11. The ejector pin feed apparatus 16 includes guide posts 18. The front ends of the guide posts 18 are fixed to the movable platen 11, and a cross head 41 is fixed to the rear ends of the guide posts 18. A ball nut 42 is fixed to the cross head 41.
The inner circumferences of the ball nut 42 is screw-engaged with a ball screw shaft 43. The ball screw shaft 43 is supported at its front end by an unillustrated bearing such that the ball screw shaft 43 is rotatable with respect to the movable platen 11.
Ejector rods 27 are fixed to the front end of the cross head 41. Thus, by rotating the ball screw shaft 43, the cross head 41 can be advanced and retracted so as to advance and retract the ejector rods 27.
The ejector pin feed apparatus 16 is operated by a servomotor 28, which is drivingly connected to the ball screw shaft 43 via a belt transmission mechanism 46. The belt transmission mechanism 46 comprises a pulley 32 attached to the output shaft of the servomotor 28, a plurality of pulleys 33 attached to the front ends of the ball screw shaft 43, and a timing belt 34 wound around the pulleys 32 and 33 with tension.
Accordingly, when the servomotor 28 is driven, the rotational motion of the servomotor 28 is transmitted to the ball screw shaft 43 via the belt transmission mechanism 46. The ball screw shaft 43 and the ball nuts 42 convert the rotational motion into a linear motion, so that unillustrated ejector pins are advanced and retracted.
However, in the above-described conventional ejector apparatuses, a considerably large tension must be applied to the timing belt 34 in order to push a molded product out of the movable die for perfect separation. In addition, since the ejector pins must be repeatedly advanced and retracted, and such a repeated operation places the ejector apparatus under severe conditions, the durability of the apparatuses deteriorates.
That is, since a large tension must be applied to the timing belt 34, an eccentric load acts on the ball screw shafts 24 (FIG. 1) and 43, thereby decreasing the efficiency of the ball screw shafts 24 and 43, increasing the load of the servomotor 28, and shortening the service life of the ball screw shafts 24 and 43.
Also, the resultant elongation of the timing belt 34 deteriorates accuracy in controlling the ejector apparatus, and causes breakage and wear of the timing belt 34. Therefore, the timing belt 34 must be exchanged for a new one within a relatively short period of time.
Moreover, dust is produced due to wear of the timing belt 34 and dirties the area around the injection molding machine.
The durability of the timing belt 34 can be improved by increasing the hardness of the timing belt 34. In this case, however, noise is generated when the ejector apparatus is operated at high speed. Also, a chain, a rack and pinion, or the like can be used instead of the timing belt 34 so as to enhance durability. In these cases, however, the ejector apparatus cannot be operated at high speed because of poor control accuracy and generation of play. In addition, lubrication is required, and the peripheral area of the ejector apparatus is therefore dirtied due to lubricant.